1. Field of the Invention
This invention relates to microsensor systems and to a micropolarimeter in which the micropolarimeter is designed to measure polarization states of light beams. The micropolarimeter is the central component in the following applications:
Ellipsometry for thin film diagnostics PA1 Laser beam diagnostics PA1 Diagnostics for polarization depending properties in optical fibers PA1 Analysis of optically active materials.
2. Description of the Background Art
A well established method for characterizing surfaces and thin films is ellipsometry. A typical ellipsometer uses discrete polarization optical components in various combinations to determine the polarization state of light in terms of measured Stokes parameters. A detailed description is to be found in R. M. A. Azzam, N. M. Bashara, "Ellipsometry and Polarized Light", 1988, North Holland, Amsterdam.
Ellipsometry working principle is based on utilizing the variation in the intensity of the light detected as a function of the polarization ellipse parameters. Typically the polarizer is fixed and the analyzer is synchronously rotated about the light beam axis at a constant angular velocity, which is shown in various publications like IBM Technical Disclosure Bulletin 18 (1975) 2031.
The detector signal will exhibit periodic variations and is Fourier analyzed. Using the Fourier coefficients together with the known incident angle and the polarizer setting the ellipsometric angles are then derived.
For instrument operation precise mechanical motion of components is required limiting the response time of the instrument and complicating its design.
Instead of rotating components some polarimeters use modulation techniques modulating the phase and/or the azimuth of the beam. The analysis of the data obtained is, however, more complicated and expensive lock-in amplifiers and modulators are needed.
U.S. Pat. No. 4,585,348 discloses an ellipsometer having no moving parts and a minimum number of optical components. This instrument consists of a static photometric polarimeter and measures statically only the reflected intensities whose polarization lie, respectively, parallel and perpendicular to the plane defined by the incident and reflected beams. However, this instrument determines only the quotient of the two polarization states which is not sufficient to determine stokes parameters. For a complete determination of the Stokes parameters a second measurement is necessary.
In another kind of polarimetric systems moving or rotating elements are avoided by using multiple detector systems. IBM Technical Disclosure Bulletin 19 (1976) 1487 shows an arrangement using four beam splitters and four detectors which allows complete determination of the polarization state of light reflected from a sample directly in terms of measured Stokes parameters. The adjustment of the four independent beams, however, is very complicated and excludes the construction of a compact, small and fully-enclosed measuring apparatus.
U.S. Pat. No. 4,158,506 discloses a realtime polarimeter with pulse mode. A broadened beam enters six photodetectors with four of the polarizers being tilted in steps of .pi./4 and two of them form an angle of .pi./2 being arranged behind a .lambda./4 plate. By electronically adding and subtracting all the four Stokes parameters may be determined. However, due to the dimensions of conventional discrete polarizers the beam has to be strongly broadened which renders the characterization of small areas on a sample impossible without using additional optical elements.
Systems with multiple detectors have the problem of having only limited number of independent channels. Therefore noise reduction by carrying out further independent measurements similar to those in arrangements with rotating elements is not possible. The measuring accuracy thus remains far below the standard accuracy of conventional polarimeters and ellipsometers with rotating elements. Adding additional channels seems to be possible in principle, but unreasonably increases the technical effort and the costs.
It is the object of the present invention to provide a miniaturized measuring tool and a measuring concept which overcome the disadvantages of the prior art concepts and allow the use in on-line process control. This object and further positive effects are achieved by the invention as claimed.